Cracking sound from amp - help

[Brit01]

Hi guys,

Getting an occasional worrying crack/pop noise coming through my amp maybe once or twice an evening. Just on left channel.

It's a 1992 Carver TFM-55. One of the rails has been replaced with new power transistors (before I bought it).

Since then I've replaced most of the power caps, some differential transistors and some resistors.
Sounds very good with the new parts but bit worried now.

Maybe a sign of one of a power transistor going?
Or maybe an old mylar cap going?

Or some old Carbon Comp. resistors?

[Jan Dupont]

Or a bad solder

[keantoken]

Originally posted by Brit01
Hi guys,

Getting an occasional worrying crack/pop noise coming through my amp maybe once or twice an evening. Just on left channel.

It's a 1992 Carver TFM-55. One of the rails has been replaced with new power transistors (before I bought it).

Since then I've replaced most of the power caps, some differential transistors and some resistors.
Sounds very good with the new parts but bit worried now.

Maybe a sign of one of a power transistor going?
Or maybe an old mylar cap going?

Or some old Carbon Comp. resistors?

Does sound like maybe some solder/connection coming loose because of thermal expansion (possibly of the new output devices - could have been a bad solder job).

- keantoken

[Brit01]

you may be right as it cracks loudly after warming up some 30 minutes.

Now the issue is where. Narrowed down to the right channel at least and I'm very familiar with the circuit as I've worked on it a great deal.

I'll be replacing some diodes and resistors in a few weeks so I'll give it a good look over.

[sivan_and]

Check for any DC excursions at the output

[djk]

"Or maybe an old mylar cap going?"

I suggest you replace the input coupling cap.

I've had the plating on the copperweld leads fail and it acts as you described.

Do not try an re-solder the original, replace it.

"Or some old Carbon Comp. resistors?"

They're carbon film type, and they can have the same copperweld lead issue. I would replace them too.

I would also replace the bias pot. If it goes open for an instant there will be a loud crack, and you could blow the amp and your speakers.